394 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



a discbarge from a motor center; (0) the motor impulse travels along motor tracts in the brain; (7) 

 along the motor nerve and, it may be, spinal cord, and, finally, (8) the mnscle is innervated. The 

 process is ])robably an acquired cerebial reflex, not accom])anied by consciousness. The stimulus 

 is indeed perceived, but probably not before the motor impulse has been discharged. The 

 stimulus causes two sorts of cerebral changes, the discharge of the motor impulse, and changes in 

 the cortex, which are accompanied by consciousness. But, contrary to the views of most physi- 

 oh)gists, we think the movement does not follow on changes in consciousness, but is simultaneous 

 with or actuallj' prior to them. What volition is concerned in the process precedes the reaction 

 and consists in preparing the motor impulse, which is reflexly discharged.' 



The conditions on which the durati(m of tlie reaction depends are partly such as relate to the 

 subject reacting and partly such as relate to the stimulus. Some subjects react more quickly tlian 

 others, and this difference in time must represent real differences in the nervous system. The 

 personal difference in reacting has not yet been adequately investigated.^ Observations which we 

 have made indicate that the reaction-time is shorter for women than for men, and for Americans 

 than for Germans. The reaction-time is said to be longer in childhood and in old age. We have, 

 however, found a normally short reaction in a child of o and an unusually short and regular 

 reaction in a man of 65. We have found the reaction-time to be lengthened in certain diseases of 

 the nervous system, and the test (especially in unilateral diseases in which the reacting hand or 

 foot and the point of application may be varied) might prove useful in diagnosis, more especially 

 in indicating ])rogression or recovery. We have found the times of mental processes such afe 

 perception, volition, memory, association, etc., to vary more in different individuals than the times 

 of the simple reaction, and these may i)rove useful not only in diagnosis of disease, but in scientific 

 pedagogy and in directing the ordinary conduct of life. Our experiments on personal differences 

 are not completed, and will not be treated in tliis paper. 



In the same individual the duration of the reaction time and of mental processes differs at 

 different times. Owing, however, to the reflex nature of the reaction its length is not greatly 

 affected by the condition of the observer, the time of day, the number of reactions already made, 

 nor the amount of practice. These factors, and esi)ecially the effects of attention, we shall consider 

 in view of our own results. It may here be stated that in our experiments the mean variation 

 of a reaction from the series to which it belongs was usually less than 0.01 second, and the mean 

 variation of series made on different days was also usually less than 0.01 second. 



The length of the reaction-time is cleaily influenced by the nature of the stimulus and the 

 point of its api)lication. The reaction-time is about 0.025 second longer for light than for sound 

 and touch. This may be due to the greater time required for converting the physical motion into 

 a nervous impulse in the retina, where a chemical in'ocess is supposed to take place. It may also 

 be due to the cerebral reflex being less perfect, reflex and automatic movements being made more 

 readily in answer to sounds and touches than to lights. We have found the reaction on touch 

 shorter than on electric stimulation. The reaction-time becomes shorter as the intensity of the 

 stimulus is increased, though the dift'erence is not great except in the case of very weak stimuli. 

 The area of the stimulus probably only affects the length of reaction in so far as it alters the 

 intensity. The quality of stimuli of the same intensity (e. g., different colors or noises) does not 

 appreciably alfect the length of the reaction. 



' It is not necessary to repeat in this place references to the somewhat extended literature on reaction-time and 

 the velocity of the nervous impulse; cf. for these Hermann, iu his Handliuch der Pliysiologie. A^ol. II, and Exuer, in 

 the same work, Vol. Ill, Leipzig, 1879; Wuiult, Grundziige der physioloKischeu Psyihologie, 4th edition, Leipzig, 

 1893; Cattell, Philosophischi' Stiidien, Vol. Ill, auilMind, Vol. XI, 1886; Dumreicher, Zur Messung diT Reactionszeit, 

 Dissertation, Strasslinrg, 1889; .lastrow, Tlie Time Kelatious of Mental Phemiracna, New York, 1890, and the 

 Catalogue of the Surgeon-General's Library. 



-It is, indeed, the case that the whole (juestion of reaction-time has had its origin in the" personal equation of the 

 astronomer. But the problems, though often confused, are quite distinct. The astronomer watches the star as it 

 crosses the field of his telescope and records as nearly as he can the instant at which it passes the central thread. 

 In such a case a reaction may perhaps be said to take place, but the personal equation of the astronomer depends 

 not on the duration of the reaction but on the time at which the ])rocess is initiated, and may be as great as one 

 second. It seems likely that the astronomer could greatly reduce his personal equation by adopting the methods of 

 the psychologist. If the star passed behind a screen and emerged as it passed the meridian, the observer could not 

 liave a negative personal equation, and the probable error of a single observation might be reduced to 0.01 second. 



